Recent interest in the study of aging has produced sufficient evidence to conclude that there are age-related declines in a wide variety of neuromuscular variables. Previous studies from these investigators have indicated that the aging neuromuscular system is characterized by distinct adaptations that may occur in response to a deteriorating neuromuscular apparatus. Current literature suggests an adaptive plasticity at the neuromuscular level in both primates and humans. Specifically, given the appropriate conditioning strategies, humans and primates have been shown to be successful at either increasing or decreasing the gain of the spinal stretch reflex. The purpose of these experiments will be to systematically examine the function of the elderly neuromuscular system with respect to the following: 1) the gain of the H-reflex in young and elderly subjects under varying environmental conditions and how this relates to static and dynamic stability; 2) presynaptic inhibition levels in young and elderly subjects under varying environmental conditions and how this relates to static and dynamic stability; and 3) recurrent inhibition levels in young and elderly subjects under varying environmental conditions and how this relates to static and dynamic stability. Finally, this proposal will investigate interaction of vision and task complexity in mediating these differences. A double stimulation technique will be used to assess presynaptic inhibition and recurrent inhibition in young and elderly subjects in different body postures. The modulation of these spinal parameters in different body postures will be correlated with static and dynamic stability. In the final year of this proposal, short- and long-term intervention programs will be implemented to improve these spinal control parameters and, ultimately, static and dynamic balance in elderly subjects.